This invention relates to a radio frequency generator for use with a variable impedance load, and in particular to an electrosurgical instrument having a monopolar electrode unit incorporating such a generator, the operating frequency of the generator being typically in excess of 5 MHz.
A known electrosurgical system comprises a handpiece, a monopolar electrode unit having a single treatment electrode projecting from the handpiece, a generator unit, and a cable coupling the generator unit to the handpiece. Such systems are commonly used for various types of electrosurgery. Normally, a conductive pad is applied to the body of the patient and connected to a return terminal of the generator unit to provide a return path for electrosurgical currents. Disadvantages of this arrangement include the localisation of electrosurgical currents in tissue in the region of the return pad and, particularly at higher frequencies, the unpredictability of reactive components created by the cable between the generator and the handpiece, leading to unpredictable voltage levels at the electrode.
These disadvantages are overcome at least in part by an instrument described in the applicant's International Application No. PCT/GB96/02577 which discloses an electrosurgical instrument comprising a handpiece, a monopolar electrode unit having a single treatment electrode projecting from the handpiece, and a radio-frequency generator within the handpiece, the generator having a single direct radio-frequency output connection, which output connection is coupled to the electrode. The generator is otherwise isolated from external elements. In particular, the generator has no other direct radio frequency output connection to, for example, an earthed element or to a return pad. By providing the generator within the handpiece, unpredictable impedance changes due to the effects of supplying radio-frequency currents through a cable are avoided. Radio-frequency return currents pass between the patient and the generator by stray capacitive coupling via a conductive shield located around the generator.
Preferably, the operating frequency of the generator is 5 MHz or greater. The higher the frequency, the greater the attainable current level due to the reduced reactance of the return path at raised frequencies. The generator may be powered from a battery within the handpiece. This minimises radiated interference.
The presence of an electrically conductive shield around the generator minimises the variation in stray capacitance caused by the user gripping the handpiece in different ways. The shield is preferably isolated from the generator and may form a tubular handpiece body, e.g. in the form of a metallic casing, or the handpiece body may be formed of an electrically insulative material which is metallised to provide the conductive shield. Where the metallisation layer is on the outside of the handpiece body, or the handpiece body is itself metallic, the outer metallic surface is preferably covered by an electrically insulating outer layer. Provision of the shield reduces stray capacitance variations because the capacitance between the relevant generator conductors and the shield is constant, and the shield provides a conductive body of constant area capacitively coupled to the patient.
Although the shield reduces variations in stray capacitance, the variable impedance load which results from this and from the inevitable variations in load caused by changing conditions at the tissue-to-electrode interface poses considerable difficulties in maintaining energy efficiency. To a lesser degree, also it poses difficulties in preventing output device breakdown due to transient mismatches.